Chapter 23 - Chapter 23 The Evolution of Populations...

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Chapter 23 - The Evolution of Populations Chapter 23 The Evolution of Populations Lecture Outline Overview: The Smallest Unit of Evolution One common misconception about evolution is that organisms evolve, in a Darwinian sense, during their lifetimes. Natural selection does act on individuals. Each individual’s combination of inherited traits affects its survival and its reproductive success relative to other individuals in the population. However, the evolutionary impact of natural selection is only apparent in the changes in a population of organisms over time. It is the population, not the individual, that evolves. Consider the example of bent grass (Agrostis tenuis) growing on the tailings of an abandoned mine. These tailings are rich in toxic heavy metals. While many bent grass seeds land on the mine tailings each year, the only plants that germinate, grow, and reproduce are those that possess genes enabling them to tolerate metallic soils. These plants tend to produce metal-tolerant offspring. Individual plants do not evolve to become more metal-tolerant during their lifetimes. Concept 23.1 Population genetics provides a foundation for studying evolution Darwin proposed a mechanism for change in species over time. What was missing from Darwin’s explanation was an understanding of inheritance that could explain how chance variations arise in a population while also accounting for the precise transmission of these variations from parents to offspring. The widely accepted hypothesis of the time—that the traits of parents are blended in their offspring—would eliminate the differences in individuals over time. Just a few years after Darwin published On the Origin of Species, Gregor Mendel proposed a model of inheritance that supported Darwin’s theory. Mendel’s particulate hypothesis of inheritance stated that parents pass on discrete heritable units (genes) that retain their identities in offspring. Although Gregor Mendel and Charles Darwin were contemporaries, Darwin never saw Mendel’s paper, and its implications were not understood by the few scientists who did read it at the time. Mendel’s contribution to evolutionary theory was not appreciated until half a century later. The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance.
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When Mendel’s research was rediscovered in the early 20th century, many geneticists believed that his laws of inheritance conflicted with Darwin’s theory of natural selection. Darwin emphasized quantitative characters, those that vary along a continuum. These characters are influenced by multiple loci. Mendel and later geneticists investigated discrete “either-or” traits.
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Chapter 23 - Chapter 23 The Evolution of Populations...

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